Antigenic responsiveness to allergens is imparted to mast cells and basophils by specific membrane binding of allergen binding IgE. Other cells have been shown to bind ligand non-randomly, especially to microvilli (dePetris, Nature 272: 66-68, 1978). Further, cell bound IgE has been shown to survive for prolonged periods of time on the cell surface. Finally, binding of cell-bound IgE with multi-valent ligand results in rapid internalization without re-expression of both IgE ligand and its receptor. We are studying the native distribution of IgE receptors on the cell surface by two techniques and comparing their fate following ligand binding. Of especial interest is the fate of planar cell surface receptors compared to those on microvilli. In addition, the role of a pre-lysosomal compartment ("CURL") in ligand-IgE-receptor uncoupling and subsequent degradation is being investigated by double label techniques (colloidal gold-ligand and IgE-ferritin or Alpha-receptor ferritin).